Azure Virtual Network Routes

Understanding Route Tables in Azure Virtual Networks

Route tables in Azure Virtual Networks (VNets) are a fundamental component for controlling traffic flow within your VNet and to and from external networks. They allow you to define custom routes to override Azure's default system routes. This enables scenarios such as enforcing traffic inspection through network virtual appliances (NVAs) or directing traffic to specific gateways.

Default System Routes

Every subnet in an Azure VNet is automatically associated with a route table. Initially, this route table contains only the default system routes provided by Azure. These routes cover:

Local VNet traffic (e.g., between subnets within the same VNet).
Traffic to the internet via the Azure backbone.
Traffic to Azure services.

Custom Routes

You can add custom routes to a route table to influence how traffic is directed. Each custom route consists of:

Address Prefix: The destination IP address range (CIDR block) to which the route applies.
Next Hop Type: Specifies where to send the traffic. Common types include:
- VirtualAppliance: Traffic is sent to a network virtual appliance (NVA) for processing.
- VirtualNetworkGateway: Traffic is sent to a virtual network gateway for VPN or ExpressRoute connectivity.
- Internet: Traffic is sent directly to the internet.
- None: Traffic is dropped (blackholed).
Next Hop IP Address: If the Next Hop Type is VirtualAppliance, this is the private IP address of the VNet interface of the NVA.

Route Table Association and Propagation

A route table can be associated with one or more subnets. Traffic originating from a subnet will use the routes defined in its associated route table.

Route Propagation:

System Routes: Azure automatically propagates its default routes to route tables associated with subnets.
BGP Routes: If you are using VPN gateways or ExpressRoute circuits with BGP enabled, routes learned via BGP will also be propagated to associated route tables.
User-Defined Routes (UDRs): Custom routes that you define within a route table.

Creating and Managing Route Tables

You can create and manage route tables using the Azure portal, Azure CLI, Azure PowerShell, or ARM templates.

Example using Azure CLI:

            
az network route-table create --resource-group myResourceGroup --name MyRouteTable

To add a custom route:

            
az network route-table route create --resource-group myResourceGroup --route-table-name MyRouteTable --name RouteToNVA --address-prefix 10.1.0.0/16 --next-hop-type VirtualAppliance --next-hop-ip-address 10.0.0.4

To associate a route table with a subnet:

            
az network vnet subnet update --resource-group myResourceGroup --vnet-name MyVNet --name MySubnet --route-table MyRouteTable

Route Prioritization

Azure evaluates routes based on the following criteria, in order of priority:

User-defined routes (UDRs) with the most specific address prefix.
BGP routes with the most specific address prefix.
System routes with the most specific address prefix.

If multiple routes have the same most specific prefix, the order of preference is UDR, then BGP, then system routes.

Important: Ensure that your route table configuration aligns with your network security and traffic management requirements. Incorrectly configured routes can lead to connectivity issues.

Troubleshooting Routes

Use the Azure Network Watcher's IP Flow Verify and Next Hop features to diagnose connectivity issues and understand how traffic is being routed.

Common Scenarios:

Forcing traffic through an NVA: Create a route for desired traffic destinations pointing to the NVA's IP address.
Controlling egress traffic: Define routes to direct internet-bound traffic through a firewall appliance.
Isolating network segments: Use specific routes to control communication between different parts of your VNet.

Understanding and effectively utilizing Azure Virtual Network route tables is crucial for designing secure, scalable, and well-managed cloud networks.